Gas turbine engines include fans and compressor rotors having a plurality of rotating blades. Minimizing the leakage of air, such as between tips of rotating blades and a casing of a gas turbine engine, increases the efficiency of the gas turbine engine because the leakage of air over the tips of the blades can cause aerodynamic efficiency losses. To minimize this, gaps at tips of the blade are set small and, under certain conditions, the blade tips may rub against and engage an abradable seal at the casing of the gas turbine engine. The abradability of the seal material prevents damage to the blades while the seal material itself wears to generate an optimized mating surface and thus reduce the leakage of air.
Aluminum based abradable coatings that are used in fan and compressor blade outer seal applications are prone to aqueous corrosion. The coatings are porous and absorb water that subsequently dries during use. When this process is repeated, contaminants in the water concentrate and can produce a conductive and corrosive electrolyte, while water is present. The conductive water trapped within the porosity of the coating results in an increased tendency for internal corrosion or crevice corrosion. The result is that the coating becomes weaker, has reduced ductility, loses its abradable characteristics, and can spall and damage airfoils.